1. Field of the Invention
The present invention relates to a pulse coder using magnetoresistance (reluctance) elements or magnetic sensors, more particularly, to the improvement of magnetic patterns for detecting a rotational reference position.
2. Description of the Prior Art
In recent years, pulse coders using magnetoresistance elements have been developed in order to detect the rotational angle of a servo motor or the like. Such pulse coders are not significantly affected by dirt and, accordingly, offer greater reliability than pulse coders using optical systems.
A prior art pulse coder using magnetoresistance elements comprises a rotating member, such as a magnetic disk, on which magnetic patterns are formed and a fixed member on which magnetoresistance patterns are formed. The magnetoresistance patterns convert the magnetic flux from the magnetic patterns into an electrical signal. Such magnetic patterns may be roughly classified into A-phase patterns for generating an A-phase signal; B-phase patterns for generating a B-phase signal; and Z-phase patterns for generating a Z-phase signal. The A-phase signal and the B-phase signal are used for detecting the rotational angle of the rotating member, while the Z-phase signal is used for detecting the reference position of the rotating member.
The present invention relates to an improvement of a Z-phase magnetic pattern.
Such a Z-phase magnetic pattern is constructed by alternately and regularly (equidistantly) arranging strong magnetic field generating areas and weak magnetic field generating areas on part of a circumference of the rotating member. The magnetoresistance pattern on the fixed member has first and second magnetoresistance elements which are electrically connected. In this case, when the first magnetoresistance element directly faces the strong magnetic field areas, the second magnetoresistance element directly faces to the weak magnetic field areas. As a result, the output voltage obtained at the connection between the first and second magnetoresistance elements reaches a maximum peak. The reference position of the rotating member can be determined by detecting such a maximum peak.
In the above-mentioned prior art, however, other peaks of output voltage are generated at the magnetoresistance element connection every cycle of the strong magnetic field generating areas and the weak magnetic field generating areas even when the detected position of the rotating member is off from the maximum peak point. In addition, the differences between the maximum peak and the other peaks are not that large. As a result, it is difficult to distinguish the maximum peak from the other peaks, causing errors of detection of the reference position of the rotating member.